Interactions among pathogens, vectors and hosts can be greatly altered by human-caused introductions of new species. The introduction of genetically-differentiated populations of the same vector or parasite may be less detectable, but have equally important impacts. We propose here to evaluate the impacts of introductions of different genotypes or strains of vectors and parasites in an introduced system of avian malaria (Plasmodium relictum), introduced vectors (Culex quinquefasciatus) and introduced and native bird hosts in Hawaii. Introduced avian malaria has had devastating impacts on the Hawaiian avifauna, playing a major role in extinction and endangerment of a number of native species. Our ultimate goal is to develop a model of the impact of anthropogenic introductions of vector populations with different vectorial capacities to the local dynamics of the diseases they transmit. We present microsatellite and mtDNA evidence indicating that Hawaiian Cx. quinquefasciatus may be mostly comprised of Australian genotypes, with only a low incidence of the originally introduced American genotypes remaining. We also present preliminary rRNA evidence that suggests multiple strains of P. relictum may also have been introduced to the islands. We propose three parts to our project: (1) to ascertain if P. relictum in Hawaii is composed of multiple strains or genotypes, and to determine from where these strains or genotypes originated in the world and whether they have changed over time; (2) to continue our search for Cx. quinquefasciatus in Hawaii that are of American origin (i.e., the source population from which they originated based on historical records), and to examine the extent of introgression or substitution of vector populations from different geographical origins; and (3) to determine if American and Australian genotypes of Cx. quinquefasciatus differ in their ability to transmit Hawaiian and other strains of P. relictum. We propose to conduct intensive morphological and molecular surveys of mosquitoes and avian tissues from worldwide sources for comparison with Hawaiian P. relictum genotypes. Once we document the closest source population(s) of Hawaiian P. relictum we will carry out a series of reciprocal challenge experiments on Cx. quinquefasciatus vectors in order to document if there is differential vectorial competence. We will test both major genotypes of mosquitoes from mainland (Australian and American) populations, as well as the introduced, partly hybrid forms in Hawaii. We will use isolates of P. relictum from America, Australia, and Hawaii, and also from sites to be determined by our results from the survey for the geographic origin of Hawaiian P. relictum.